By Dr. Eldridge M. Mount
Adhesive bonding and surface treatment of substrates go hand in hand. However, there also is an important role of substrate design that will interact with the surface-treatment methods employed to enhance or perhaps diminish adhesion to the substrate.
These considerations go beyond the simple measure of surface-treatment level and are key to developing the optimum surface chemistry to enhance the ultimate adhesive strength developed for the many converting processes in which the substrate will be used. The surface chemistry also must be matched to the chemistry of the primer or adhesive system or the ink being used in the converting process.
The ABCs of layer design
In general, I would assume that most substrate product designs are 3-layer ABC films, where the “A” surface is the converting layer, and the “C” surface is the sealing layer. In my opinion, a symmetrical A/B/A film or perhaps an A/B two-layer film has too many product-design limitations to be considered for most situations.
While there are films with more than three, five or perhaps seven to enhance the substrate properties, the “A” and “C” layers will still have the same functionality as in the 3-layer films. The “B” and “C” layer product design and polymers will not be addressed here, just the “A” layer.
Getting oriented
Oriented-polyester (OPET) and Nylon (OPA) films are more limited in skin-resin selection than an oriented PP or HDPE film due to the more limited number of available skin layers. However, coextruded A and C skins would be beneficial simply because the skins can be formulated for web-handling and winding as well as for enhancement in converting properties.
In the case of PET films, various amorphous polyester resins, such as PETG, can be selected to enhance skin-adhesion properties. In the case of OPA films, the same is true, but the use of amorphous PA skins is far less common than for the OPET and especially for OPP films. In comparison to OPP, OPET and OPA, oriented HDPE (BIHD, OHD) films are less available today than perhaps they will be in the future.
Examining the converting skin of OPP films will illustrate the general concept of this article and can be applied to the OPET, OPA and BIHD films with suitable experimentation to optimize skin selection and treatment methods. OPP films for converting and metallization have been produced with skins of: homopolymer PP, EPcopolymer [1], BPcopolymer, EBPterpolymers, HDPE [2,3], EVOH [4], amorphous Nylon [5], to name a few. The advantages of the various skin layers are well documented in the patent literature and, in general, the skin selection is combined with various surface-treatment methods such as corona, flame and plasma to enhance the converting performance of the product design. The optimum surface treatment should be selected experimentally and will depend on the optimum surface chemistry needed for the particular converting process.
References
- All and Duncan, US Patent 4,345,005, Aug. 17, 1982
- Migliorini & Mount III, US Patent 5,194,318, March 16, 1993
- Osgood & Therrian, US Patent 4,855,187, Aug. 8, 1989
- Migliorini, US Patent 5,153,074, Oct. 6, 1992
- Migliorini & Mount III, US Patent 5,591,520, Jan. 7, 1997